Abstract:

Polycrystalline alumina and methods for manufacturing polycrystalline
alumina exhibiting improved transmission in the infrared region. In one
embodiment, polycrystalline alumina articles are formed by providing a
powder of substantially alpha phase alumina having a grain size of up to
about 1 μm, dispersing the powder in a liquid to form a slurry
comprising powdered solids and liquid, removing excess of the liquid from
the slurry to form a body, heating the body to provide a densified body,
hot isostatically pressing the densified body under conditions to provide
an article having a density of at least about 99.9% of theoretical
density, and optionally annealing the article, wherein one or more of the
annealing or heating are performed in an inert, dry gas.

Claims:

1. A method for manufacturing a transparent polycrystalline alumina
article comprising:providing a powder of substantially alpha phase
alumina having a grain size of up to about 1 μm;dispersing the powder
in a liquid to form a slurry comprising powdered solids and liquid;adding
one or more sintering aids;removing excess of the liquid to form a green
body; andheating the green body at a temperature of at least about
600.degree. C. for at least about 1 hour to provide a prefired
body;sintering the prefired body at a temperature and time in a dry
nonoxidizing gas atmosphere to provide a sintered body having a density
greater than about 95.5% of theoretical density; andhot isostatically
pressing the sintered body at a pressure of at least about 103 MPa
(15,000 psi) gas pressure at a temperature of at least about 1225.degree.
C. to provide a transparent polycrystalline alumina article having
substantially no OH.sup.- absorption band when exposed to light at
wavelengths from between about 2000 nm and about 5000 nm.

2. The method of claim 1, wherein the the polycrystalline alumina article
has a grain size of less than about 0.7 μm.

3. The method of claim 1, wherein the polycrystalline alumina article
exhibits in-line transmission values of within about 5% of those obtained
for sapphire when exposed to light at wavelengths of from about 3000 nm
to 4000 nm.

4. The method of claim 1, wherein the polycrystalline alumina article has
a density of at least about 99.9% of theoretical density and exhibits a
hardness of at least about 25 GPa and a fracture toughness of at least
about 2.6 MPa m1/2.

5. The method of claim 1, further comprising adding to the slurry one or
more sintering aids selected from the group consisting of MgO in a
concentration of between about 25 ppm and about 3000 ppm by weight of the
of the solids, Y2O3 in a concentration of between about 25 ppm
and about 3000 ppm by weight of the solids, and ZrO2 in a
concentration of between about 25 ppm and 3000 ppm by weight of the
solids.

6. The method of claim 1, wherein the article is in the form of an
infrared-transmitting window or dome and exhibits an in-line transmission
of:a) at least about 83.9% for a sample 2 mm thick when exposed to light
at a wavelength of about 3000 nm; and/orb) at least about 86.4% for a
sample 2 mm thick when exposed to light at a wavelength of about 4000 nm.

7. The method of claim 1, wherein the excess liquid is removed by a
casting method.

8. The method of claim 1, wherein the excess liquid is removed by pressure
casting.

9. The method of claim 1, wherein the green body is heated at temperatures
between about 600.degree. C. and about 1000.degree. C.

10. The method of claim 9, wherein the heated green body is sintered in an
atmosphere comprising dry H.sub.2.

11. The method of claim 1, wherein the sintered body is subjected to hot
isostatic pressing for about two hours at a pressure of at least about
103 MPa (15,000 psi) in gas at temperatures in the range of about
1225-1325.degree. C.

12. (canceled)

13. (canceled)

14. A method for manufacturing a transparent polycrystalline alumina
article comprising:providing a powder of substantially alpha phase
alumina having a grain size of up to about 1 μm;dispersing the powder
in a liquid to form a slurry comprising powdered solids and
liquid;removing excess of the liquid from the slurry to form a
body;heating the body to provide a densified body;hot isostatically
pressing the densified body under conditions to provide an article having
a density of at least about 99.9% of theoretical density; andannealing
the article in a dry inert gas atmosphere to provide a transparent
polycrystalline alumina article having substantially no OHabsorption band when exposed to light at wavelengths from between about
2000 nm and about 5000 nm.

15. The method of claim 14, wherein heating the body is conducted at a
temperature and time to achieve closed porosity in the body.

16. The method of claim 15, wherein the annealing is performed at a
temperature of about 1100.degree. C.

17. The method of claim 16, wherein hot isostatically pressing is
performed for about two hours at 103-207 MPa (15,000-30,000 psi) in gas
at temperatures between 1225-1325.degree. C.

18. The method of claim 17, wherein the article has a hardness of at least
about 25 GPa and a fracture toughness of at least about 2.6 MPa
m1/2.

19. The method of claim 16, wherein the polycrystalline alumina article
exhibits an in-line transmission of at least about 83.9% for a sample 2
mm thick when exposed to light at a wavelength of about 3000 nm and/or an
in-line transmission of at least about 86.4% for a sample 2 mm thick when
exposed to light at a wavelength of about 4000 nm.

20. The method of claim 16, wherein the article is annealed for at least
28 hours.

21. The method of claim 14, wherein the polycrystalline alumina article is
in the form of an infrared-transmitting window or dome and has a grain
size of less than about 0.7 μm.

22. The method of claim 14, wherein the polycrystalline alumina article
exhibits in-line transmission values of within about 5% of those obtained
for sapphire when exposed to light at wavelengths of from about 3000 nm
to 4000 nm.

23. The method of claim 14, further comprising adding to the slurry one or
more sintering aids selected from the group consisting of MgO in a
concentration of between about 25 ppm and about 3000 ppm by weight of the
of the solids, Y2O3 in a concentration of between about 25 ppm
and about 3000 ppm by weight of the solids, and ZrO2 in a
concentration of between about 25 ppm and 3000 ppm by weight of the
solids.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is a divisional application of U.S. application
Ser. No. 11/240,069, filed on Sep. 30, 2005, which claims the benefit of
the filing date of U.S. Provisional Patent Application No. 60/522,438
filed Oct. 1, 2004, the disclosures of which are hereby incorporated
herein by reference.

FIELD OF THE INVENTION

[0003]Embodiments of the invention relate to polycrystalline alumina (PCA)
articles, for example, missile domes, missile windows and other infrared
transmitting windows, having substantially no absorption peaks at
wavelengths from about 2000 nm to about 5000 nm and methods of
manufacturing such articles. Other embodiments relate to polycrystalline
alumina articles having improved mechanical properties such as hardness
and fracture toughness.

BACKGROUND OF THE INVENTION

[0004]Transparent ceramics such as transparent alumina articles are used
in a wide variety of applications such as discharge lamps, supermarket
scanner windows, window plates for furnaces, and military applications
such as infrared transmitting windows, missile domes and missile windows.

[0005]As surveillance and tactical missions become more complex, there is
a need to increase the performance of infrared (IR) systems to provide
higher quality and higher resolution imagery. Typically, IR systems on
tactical missiles are protected by optically transparent windows or
domes, which are exposed to a broad range of environmental and operating
conditions. The increasing sensor performance requires commensurate
improvements in window performance, so that the window does not limit the
imaging capability of the sensors. Missile domes are one of the most
demanding applications for ceramics. Severe aero-thermal heating occurs
as the missile accelerates to its programmed velocity, which necessitates
the use of a material with excellent thermal shock resistance. Good
thermal shock resistance is a function of the material's intrinsic
physical properties and the extrinsic property of high strength coupled
with high Weibull modulus--a combination that avoids premature dome
failure. All of these physical properties must accompany a ceramic that
is transparent over a broad range of wavelengths. The domes, therefore,
require a wide band gap ceramic material in either the single crystal or
polycrystalline form.

[0006]Water droplet impact damage is another consideration for materials
exposed to the speeds and altitudes associated with supersonic flight. In
addition, abrasion erosion due to sand particles can be a significant
problem. The use of ceramics as windows and domes in IR missile systems
requires extended service life without degradation of performance, more
robust window and dome survivability (e.g., increased scratch resistance,
strength, and thermal shock resistance), and low-cost manufacturing
processes.

[0007]Historically, IR transmitting windows and domes have been fabricated
from single crystal and large grain (>10 μm) ceramics including,
for example, MgF2, MgAl2O4, AlON and single crystal
Al2O3 (sapphire). As noted above, thermal shock resistance is
an important consideration, and due largely to its intrinsic properties
of high thermal conductivity and low thermal expansion, alumina has
higher thermal shock resistance than other candidate dome materials.

[0008]While all of these ceramics have been used successfully in this
demanding application, each has its limitations in terms of optical and
mechanical properties and price/performance trade-offs. Single crystal
materials can be expensive and time-consuming to manufacture and machine
into the appropriate shapes, and large grain polycrystalline materials
often do not have adequate mechanical and thermomechanical properties to
meet the increasing demands of hypersonic flight.

[0009]As surveillance and tactical missions become more vital and missile
speeds increase, there is a need to increase the performance of infrared
(IR) systems to provide higher quality imagery. Increasing missile
velocities coupled with higher sensor performance requires commensurate
improvements in window and dome performance, including hemispherical and
ogive (aerodynamic) shapes. An ogive shape enables some combination of
increased range, speed, and payload because of reduced drag. The ogive
shape also offers improved rain impact resistance and sand erosion
resistance and a greater unvignetted field of view. A method that will
allow near-net shape processing of an aerodynamic dome using a material
that has the benefits of sapphire is desired.

[0010]High-density, large grain-sized PCA material, routinely manufactured
commercially for lamp envelopes and orthodontia brackets, is not suitable
for dome applications since it is translucent due to birefringent
scattering of light as it traverses through the many grain boundaries.
This intrinsic property results from the hexagonal crystal structure of
alumina. The objectionable birefringence is eliminated as the grain size
approaches the wavelength of light.

[0011]Thus, there is a need to provide novel transparent polycrystalline
alumina articles exhibiting improved mechanical and optical properties,
particularly for use in missile domes and windows, as well as a need for
improved processes for forming such articles. There is a further need to
provide such articles that do not exhibit absorption peaks in the
transmittance range required. For applications requiring transmittance in
the mid-wave infrared (MWIR) the articles should have substantially no
absorption peaks from about 2000 nm up to about 5000 nm.

SUMMARY OF INVENTION

[0012]One aspect of the present invention relates to a method of
manufacturing alumina articles comprising providing a powder of
substantially alpha phase alumina having a grain size of up to about 1
μm; dispersing the powder in a liquid to form a slurry comprising
powdered solids and liquid; adding one or more sintering aids; removing
excess of the liquid to form a green body; heating the green body at a
temperature of at least about 600° C. for at least about 1 hour to
provide a prefired body; sintering the prefired body at a temperature and
time to provide a sintered body having a density greater than about 95.5%
of theoretical density; hot isostatically pressing the sintered body at a
pressure of at least about 15,000 psi gas pressure at a temperature of at
least about 1225° C. to provide a densified body; and optionally
annealing the densified body at a temperature of at least about
1100° C. for at least about 28 hours to provide the
polycrystalline alumina article, wherein one or more of sintering the
green body and annealing the densified body are performed in a dry inert
gas.

[0013]In one or more embodiments, the annealing step is performed in a dry
gas. In other embodiments, an annealing step is not performed. According
to one or more embodiments, the polycrystalline alumina article has a
density of at least about 99.9% of theoretical density and exhibits a
hardness of at least about 25 GPa and a fracture toughness of at least
about 2.6 MPa m1/2.

[0014]In one or more embodiments, the method further comprises adding to
the slurry one or more sintering aids selected from the group consisting
of MgO in a concentration of between about 25 ppm and about 3000 ppm by
weight of the solids, Y2O3 in a concentration of between about
25 ppm and about 3000 ppm by weight of the solids, and ZrO2 in a
concentration of between about 25 ppm and 3000 ppm by weight of the
solids. The method may further comprise shaping the article into the form
of a window or a dome exhibiting an in-line transmission of a) at least
about 83.9% for a sample 2 mm thick when exposed to light at a wavelength
of about 3000 nm; and/or b) at least about 86.4% for a sample 2 mm thick
when exposed to light at a wavelength of about 4000 nm.

[0015]In certain embodiments, the excess liquid is removed by a casting
method. In specific embodiments, the excess liquid is removed by pressure
casting. In other specific embodiments, the green body is heated at
temperatures between about 600° C. and about 1000° C. In
one or more embodiments, the heated green body is sintered in an
atmosphere comprising dry H2.

[0016]In a specific embodiment, the heated green body is subjected to hot
isostatic pressing for about two hours at a pressure of at least about
15,000 psi in gas at temperatures in the range of about 1225-1325°
C. In other specific embodiments, the article is annealed in dry inert
gas at a temperature of about 1100° C. for at least 28 hours.

[0017]In another embodiment, a method for manufacturing a polycrystalline
alumina article comprises: providing a powder of substantially alpha
phase alumina having a grain size of up to about 1 μm; dispersing the
powder in a liquid to form a slurry comprising powdered solids and
liquid; removing excess of the liquid from the slurry to form a body;
heating the body to provide a densified body; hot isostatically pressing
the densified body under conditions to provide an article having a
density of at least about 99.9% of theoretical density; optionally
annealing the article; and shaping the article in the form of a window or
dome, wherein one or more of heating the body and annealing the article
are performed in a dry inert gas atmosphere. In a specific embodiment,
heating the body is conducted at a temperature and time to achieve closed
porosity in the body.

DETAILED DESCRIPTION

[0018]Before describing several exemplary embodiments of the invention, it
is to be understood that the invention is not limited to the details of
construction or process steps set forth in the following description. The
invention is capable of other embodiments and of being practiced or being
carried out in various ways.

[0019]Embodiments of the invention provide improved polycrystalline
alumina (PCA) articles, which are particularly useful in military
applications such as infrared transmitting windows, missile domes and
missile windows. According to embodiments of the invention,
polycrystalline alumina is provided that exhibits aerothermal performance
comparable to or better than sapphire. Polycrystalline alumina also
offers the opportunity for powder-based manufacturing, which is generally
lower cost than processing associated with single crystal sapphire.
Polycrystalline alumina articles according to embodiments of the present
invention exhibit high strength, high hardness, high toughness and high
Weibull modulus all being a function of the PCA microstructure. The
strength and Weibull modulus can be improved to values comparable to
sapphire by processing polycrystalline alumina to a fine grain size, and
the resulting PCA material will have high thermal shock resistance.

[0020]The term "domes" typically refers to dome shapes made from these
materials which are placed on the front end of missiles to protect the IR
detectors which are designed to fly at very high speeds. Exemplary dome
shapes include hemispheric and ogive (aerodynamic) shapes.

[0021]One aspect of the present invention relates to polycrystalline
alumina articles. Another aspect of the invention relates to methods for
manufacturing such polycrystalline alumina articles.

[0022]In one aspect, the invention discloses a method of producing PCA
articles having submicron-size grains and substantially no absorption
when exposed to light at wavelengths from about 2000 nm to about 5000 nm.

[0023]As a starting material, commercially available Al2O3
powders having crystallite sizes less than about 0.5 quadraturem can be
used. These powders usually contain transitional phases such as gamma or
delta in addition to alpha, the high temperature stable hexagonal phase.
According to one embodiment, submicron powder that is essentially 100%
alpha phase is used as a starting material. In addition, powders for
optical ceramics must be high purity, such as at least 99.9% pure, more
preferably, at least 99.97% pure. TAM-DAR powder, available from Taimei
Chemicals Co., Ltd., Japan, is an example of a suitable starting material
having high alpha phase content and high purity.

[0024]The powders used as starting materials are typically somewhat
agglomerated. The individual crystallites are slightly bonded in some
cases forming a larger particle than desired. The bonding is a natural
consequence of calcining the precursor salt, ammonium aluminum carbonate
hydroxide (NH4AlCO3(OH)2), not only to decompose it, but
also to transform it to 100% alpha phase. The agglomerates are of two
types: soft agglomerates and hard agglomerates. Soft agglomerates can be
broken apart by some mechanical process such as sonification or ball
milling. Hard agglomerates are well sintered, and usually cannot be
broken into their individual crystallites. Filtering or sedimentation can
eliminate them. According to one embodiment of the invention, hard
agglomerates are eliminated and soft agglomerates are broken apart by the
methods described above.

[0025]According to one or more embodiments, the powder is dispersed to
form a water-based slurry. Organic-based slurries may also be used.
Examples of suitable organic liquids include, but are not limited to,
alcohols and aldehydes such as methanol, ethanol, propanols, and
benzaldehyde. A small amount, for example, less than about 1-3% by weight
of a dispersant can be added to the slurry. Suitable dispersants include,
for example, organic acids (benzoic acid and hydroxybenzoic acids,
linoleic acid, citric acid which can be obtained from Sigma-Aldrich Co.)
or acrylics, acrylic polymers, and acrylic co-polymers. Preferred
dispersants include hydroxybenzoic acid, the acrylic polymers Darvan 821
and Darvan C sold by RT Vanderbilt Co. and Narlex sold by National Starch
and Chemical Company, and sorbitan trioleates including Span 85 from ICI
Group. The best results were obtained with hydroxybenzoic acid--alcohol
solutions and acrylic copolymer--water solutions. Addition of the
dispersant is not essential but it assists in keeping the powder in
suspension during the subsequent forming operation. The slurry at this
point is about 15 volume percent solids and has a very low viscosity,
which is substantially equivalent to milk. The slurry can also be
concentrated by removing some of the liquid to increase the solids
loading up to about 60 volume percent. Sintering aids such as MgO,
Y2O3 and/or ZrO2, in amounts ranging from 25 ppm to 3000
ppm by weight may be added to the slurry to improve the final properties
of the articles. Each of these additives may be added alone or in
combination with one or more of the other additives.

[0026]Various shapes have been formed by filter casting or pressure
casting. The slurry was poured into a funnel covered by 0.2 μm filter
paper upon which the slurry was collected. In filter casting, vacuum was
pulled underneath the funnel and the water vehicle flowed through the
filter into a container. A semi-solid cake slowly formed above the filter
paper. After the last of the free water was pulled through, the cake was
allowed to air dry prior to removal from the casting device.

[0027]Alternatively, one can use pressure casting, which is quite similar
to filter casting, except that air pressure is applied above the slurry
to assist in forming the cake. A combination of both vacuum and pressure
may also be used to form the cakes. Care must be taken in removing the
samples to avoid bending or cracking. A person skilled in the art will
appreciate that a number of additional ceramic processes could be
utilized in forming desired shapes. These include, but are not restricted
to, gel casting, freeze casting, slip casting, centrifugal casting,
extrusion, injection molding, and isostatic pressing. The invention is
not limited to a specific forming method. However, the dry density of the
sample should be over 50% and preferably over 55% of theoretical density
after forming.

[0028]According to one or more embodiments, the dried samples are then
prefired by heating at temperatures from about 600° C. to about
1000° C., holding the samples for one to twelve hours or longer
and oven cooling (natural cooling curve after switching off the power).
All temperatures within that range produced transparent polycrystalline
alumina compositions but a peak temperature of 800-900° C. is
presently preferred.

[0029]In one or more embodiments, the prefired samples are then ready for
densification, which may be performed in two steps. According to one
embodiment, the first densification step includes sintering. In this
step, the goal is to densify to greater than about 95.5% but less than
about 100% of theoretical density. It is desirable to reach closed
porosity, that is, a condition where gases or liquids have no direct path
to reach internal pores. According to one or more embodiments, a second
densification step including hot isostatic pressing (HIPing) is performed
to achieve closed porosity with minimal grain growth. HIPing enables the
production of articles with near 100% theoretical density at the smallest
grain size possible. Sintering and densification also results in the
decomposition of volatile residual chemical species such as carbonates
remaining from the powder synthesis step. It is desirable that such
species be decomposed and volatilized from the sample prior to reaching
closed porosity. It is also important to prevent other contaminants such
as hydroxyls from bonding with the Al2O3 lattice. Decomposition
of species and prevention of hydroxyl bonding is important to avoid
undesirable absorption peaks in the infrared spectrum between about 2000
nm and 5000 nm. In the event that there are remaining absorption peaks,
some absorption peaks can be removed subsequently by annealing in a
controlled atmosphere. By varying the temperature, time, heating rate,
intermediate hold times, atmosphere, and the addition of sintering aids,
transmittance can be maximized and absorption peaks can be removed during
the sintering process or post-sinter annealing.

[0030]HIPing is accomplished by stacking the samples in an Al2O3
crucible, which may be separated by thin sheets of molybdenum metal and
surrounded by coarse high purity Al2O3 setter powder. The metal
sheet prevents the samples from sticking together and the setter powder
insures the purity of the samples during HIPing by absorbing any furnace
impurities. Under some conditions, the molybdenum separators and setter
powder may not be necessary during HIPing. HIPing is accomplished in the
examples described below by heating the samples to 1250-1300° C.
in 15,000-30,000 psi Argon and holding typically for 2 hours, though
other HIP conditions could produce similar results. HIPed samples are
greater than about 99.90% and preferably greater than about 99.93% of
theoretical density.

[0031]It will be understood by those skilled in the art that the heating
steps described above (prefire, sinter, HIP and annealing) may be
combined into a continuous heating cycle to achieve the same result. The
invention is not limited to a particular heating schedule or series of
heating steps, so long as the desired high transmittance and lack of
absorption peaks in the infrared range of about 2000 nm to about 5000 nm
are achieved.

[0032]After the samples have been formed into densified articles, such as
domes or windows, which may be in a variety of shapes such as
hemispherical or ogive domes, or flat or curved windows, the samples are
then ready for machining and polishing. According to one embodiment of
the invention, the samples are finished to either 0.8 mm or 2 mm
thickness. The thinner samples are used for comparative studies while the
thicker samples represent a thickness closer to the application
thickness. The thicker samples gage the acceptability of the material for
infrared applications. If the samples contain an OH absorption band, such
bands may be removed by annealing at about 1100° C. for an
extended time, such as, for example, 28 hours. The annealing may be
performed in a dry inert gas atmosphere. We have studied the OH diffusion
rate "D" experimentally, and we found the diffusion rate to be
D=6×10-3 cm2/sec. An understanding of the diffusion rate
permits calculation of the necessary annealing time for various sample
thicknesses.

[0033]In one or more embodiments, the annealing step may be eliminated. In
certain embodiments, the first sintering atmosphere governs the presence
of the OH band. According to one or more embodiments, some sintering
atmospheres do not result in an OH absorption band. For example, as will
be seen from the Examples below, sintering in a dry hydrogen atmosphere
produces an article that does not exhibit an OH absorption band.

[0034]Without intending to limit the invention in any manner, the present
invention will be more fully described by the following examples.

EXAMPLE 1

[0035]Fifty (50) grams Taimei TM-DAR powder, 100 ml distilled water, and
500 g high purity alumina milling media were measured into a 500 ml
polyethylene jar. To this was added less than about 1 weight percent
acrylic copolymer dispersant and appropriate amounts of MgO and
Y2O3 to yield approximately 100 ppm by weight of each sintering
aid in the cast sample. The jar was placed on a roller mill for several
hours, after which the milling media was filtered out and the slurry was
poured into several 100 ml polyethylene containers. The slurry portions
were allowed to sit for 1-4 days and then filter cast to form disks about
35 mm diameter and 1-3 mm thick. The disk was prefired by heating in air
at 100° C./hour to 700° C., held for 2 hours followed by
furnace cooling. The prefired disk was placed on an Al2O3 tray
with a small amount of coarse high purity Al2O3 powder
separating the disk from the tray. The disk was heated in a wet hydrogen
atmosphere to 700° C. in 1 hour followed by a reduced heating rate
to approximately 1300° C. in 2 hours. The furnace was held at
approximately 1300° C. for 2 hours followed by a 2 hour cooling
cycle. The sample was HIPed at 1300° C. for 2 hours at 30,000 psi
in Argon. The sample was polished to 0.8 mm thickness, and measured for
optical transmission. The in-line transmission was measured from about
400 nm to 2000 nm using a Perkin Elmer Lambda 900 Spectrophotometer and
from 2000 nm to 5000 nm using a Brucker 66IFS FTIR. As noted in Table 1,
the transmission was excellent over a broad range of wavelengths and
particularly high at 4000 nm. The spectrum did contain a broad OH
absorption from about 2700 nm to about 3700 nm.

EXAMPLE 2

[0036]A prefired disk was prepared as in Example 1, except an appropriate
amount of MgO was added to the slurry to yield approximately 300 ppm MgO
and 100 ppm Y2O3 by weight in the sample. The prefired sample
was again sintered in wet H2, but at 1275° C. for 2 hours.
The disk was HIPed as described in Example 1. However, the sample was
ground and polished to 2 mm thickness. The in-line transmittance of the
sample was measured as in Example 1. Initially, the sample showed OH
absorption, so it was annealed in flowing dry N2 at 1100° C.
for 58 hours. The OH absorption was almost entirely eliminated. The
transmittance values are shown in Table 1.

EXAMPLE 3

[0037]A slurry similar to that used in Example 2 was prepared. The disk
was prepared by pressure casting at 40 psi, and was prefired at
800° C. The prefired sample was sintered in the same manner as
Example 2 and HIPed the same way as in Examples 1 and 2. It was
subsequently annealed at 1100° C. in flowing dry N2 for 50 h.
Transmittance values, measured the same way as in Examples 1 and 2, for
the polished 2 mm thick disk are shown in Table 1.

EXAMPLE 4

[0038]This sample contained approximately 300 ppm of MgO and 300 ppm of
ZrO2 (both by weight) as sintering aids. It was prepared by filter
casting as in Example 1 and prefired at 800° C. It was sintered
the same way as Example 2 and HIPed the same way as Example 1. The sample
was not annealed in N2, and it showed OH absorption. Transmittance
values (measured in accordance with Examples 1-3) for the 0.8 mm thick
sample are shown in Table 1.

EXAMPLE 5

[0039]This sample was prepared the same as Example 1 but it contained only
300 ppm MgO and it was prefired at 800° C. It was sintered the
same as Example 2. However, it was HIPed at 1250° C. for 2 hour at
30,000 psi. It was annealed in flowing dry N2 for 28 hours at
1100° C. The transmittance (measured as in Examples 1-4) of this
0.8 mm thick sample is shown in Table 1.

EXAMPLE 6

[0040]This sample was prepared the same as Example 1, but it contained
only 300 ppm MgO and it was prefired at 900° C. for 2 hours. The
sintering was conducted at 1275° C. for 2 h in an air atmosphere.
HIPing was conducted as in Example 2. The sample contained an OH
absorption peak. The overall transmittance of this 0.8 mm thick sample
measured in accordance with Examples 1-5 is shown in Table 1.

EXAMPLE 7

[0041]This sample was prepared the same as Example 1 except it contained
only 300 ppm MgO and prefiring was conducted at 800° C. Sintering
was conducted in a dry H2 atmosphere. The prefired disk was heated
to 700° C. in 1 hour followed by heating to 1275° C. in 2
hours, and held at temperature for 2 hours before cooling in 2 hours. The
sample was HIPed as in Example 1. This sample did not have an OH
absorption. This 0.8 mm thick sample possessed the excellent optical
properties shown in Table 1 and measured the same way as in Examples 1-6.

[0042]As can be seen from Table 1, the transmittance values for Examples 2
and 3 are within about 2% of sapphire at 4000 nm. In addition, for
Examples 2 and 3, transmittance values from 3000 nm to 4000 nm are within
5% of the value of sapphire.

[0043]Total transmittance is defined as all the radiation transmitted over
a 180° arc in the forward direction. For most applications of this
material, specular transmission will be more important, but we thought it
instructive to characterize the total transmittance on selected samples.
Total transmittance was measured using a Cary spectrophotometer and the
data are reported in Table 2. The total transmittance is much higher than
the in-line transmittance in the visible region, and the spread between
total and in-line narrows with increasing wavelength. This is another
consequence of birefringent scattering, and is further proof that
although minimized, birefringence influences transmittance at wavelengths
near the grain size of the PCA.

[0044]Grain size was characterized on a number of samples. Grain size is
important as discussed earlier to minimize the effect of birefringence.
Representative grain sizes for several samples, along with their thermal
history, are shown in Table 3. These grain sizes were measured from
scanning electron microscopy (SEM) calibrated photographs by the
well-accepted lineal intercept method where the average linear intercept
was multiplied by 1.5 to correct for statistical factors.

[0045]These samples were prepared similarly to those described in Examples
1-7, but with the sinter and HIP conditions as described in Table 3 and
the sintering additives as follows. Examples 8 and 9 contained only the
MgO sintering aid (500 ppm and 300 ppm by weight, respectively), while
examples 10-12 contained both 300 ppm MgO and 100 ppm Y2O3 by
weight. The grain sizes correlate most rigorously with combined thermal
cycle. It is thought that the presence of Y2O3 contributes to a
smaller grain size for a given thermal cycle. The smaller grain sizes
give higher transmission values, particularly in the visible and low
wavelength infrared range. The presence of the ZrO2 sintering aid
was not shown to improve optical performance, but it may prove essential
as a grain growth inhibitor for some applications where transparent
alumina is exposed to high temperatures for long times.

[0046]Mechanical properties (hardness and indentation fracture toughness,
IFT) have been measured by the Vickers indentation technique employing a
1 kg load. Hardness was measured on numerous samples (examples 5, 8, and
9 from above). There was very little spread in the data, and the average
hardness was 25.1 GPa. This value is approximately 15% higher than the
value of 22 GPa reported for sapphire (D.C. Harris, Materials for
Infrared Windows and Domes: Properties and Performance, SPIE Press,
1999). Hardness is an important attribute for wear, abrasion, or impact
resistance. Indentation fracture toughness was measured on three
transparent samples (examples 5, 8, and 9 from above) and the results
showed KIc values of 2.64, 2.60, and 2.72 MPa m1/2. This is
about 30% higher than the fracture toughness of 2.0 MPa m1/2
reported for sapphire (D.C. Harris, ibid.). One high density, but not
fully transparent sample measured KIc=3.53 MPa m1/2 indicating
some microstructural modifications of this material can be extremely
tough. Toughness is important for the mechanical application mentioned
above, but also for thermal shock resistance. In addition, high
toughness, and in particular high hardness, are important attributes for
applications requiring high ballistic performance, for example, in
articles used for armor applications, such as vehicle armor, aircraft
armor and body armor.

[0047]The flexural strength was measured and the Weibull modulus was
calculated. Strength was measured in 4-point bending according to ASTM
standard C1161-02c using type A specimen bars. Bars were machined from
articles processed similarly to Examples 5, 7, and 9. Twenty (20) bars
were tested in the as-machined state, and twenty (20) bars were annealed
after machining (12 hours at 1050° C.) and then tested. The data
in Table 4 show high strength and high Weibull modulus, both of which
improve after annealing.

[0048]It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention without
departing from the spirit or scope of the invention. Thus, it is intended
that the present invention cover modifications and variations of this
invention provided they come within the scope of the appended claims and
their equivalents.